1. Field of the Invention
The invention concerns regulation control techniques for pulse-width-modulated, push-pull power converter circuits. More specifically, the invention pertains to arrangements for ensuring current balance between the power switching devices of the inverter stage of a converter circuit and for preventing DC saturation of the inverter transformer core under a variety of operating conditions.
2. Description of the Prior Art
The well-known push-pull type of inverter stage used in DC/DC power converters is attractive, because only one power switching device is conducting at any given instant of time, resulting in minimum saturation drop loss and increased efficiency. A disadvantage of the push-pull inverter approach is the tendency for inverter transformer core saturation caused by inverter components with dissimilar electrical operating characteristics. Inverter transformer core saturation, in turn, often results in converter power supply failure.
One prior approach to avoiding inverter transformer core saturation is to attempt to precisely match the electrical characteristics of the inverter power switching components. However, matching of the power components has the inherent disadvantages of increased component cost, difficulty of servicing existing units, and degradation of the critical match of component characteristics caused by temperature and aging.
The prior art teaches an electronic circuit alternative to device characteristic matching. One such prior art approach is set forth in U.S. Pat. No. 4,002,963--Hunter, issued Jan. 11, 1977. The inverter switching device dynamic current balance technique taught by Hunter involves monitoring the current level drawn by each inverter device, and, upon mismatch, altering the conduction time in each alternate half cycle of converter operations for that device drawing a current outside of the allowable reference norm.
While such a dynamic current balance approach improves push-pull converter performance and reliability, it is not effective for all converter operating conditions. The current balance function in circuits such as that taught in the above-referenced Hunter patent ceases to be effective when the conduction interval of the inverter power switching devices becomes long enough to approach overlapping conduction between the switching devices. Such overlap must be prevented, else the power switching devices will be destroyed. The conventional approach to preventing such overlap is the creation of a fixed time interval in each half cycle of converter operation wherein both inverter power switching devices are forced to the non-conducting state. This time interval is commonly referred to as the dead band interval.
Conditions such as converter input below rated low-line value, sudden increases in converter loading, or converter shutdown tend to prolong the required conduction interval of the inverter switching devices to the extent that the dead band interval must be relied on for device turn-off in each half cycle. However, under this condition, the converter's pulse width modulation control circuitry can no longer compensate for unbalanced inverter currents, resulting in the possibility of inverter transformer core saturation.
Hence, there is seen to be a need in the prior art for a means of providing push-pull converter dynamic current balance under all converter operating conditions.